Arrays of light emitting diodes (“LEDs”) are utilized for a wide variety of applications, including for ambient lighting. To achieve emission of light perceived as white light, LED arrays typically utilize a combination of red, green, and blue (“RGB”) LEDs (and, occasionally, amber LEDs), usually as a first series connection (a first “string”) of a plurality of red LEDs, a second series connection (a second string) of a plurality of green LEDs, and a third series connection (a third string) of a plurality of blue LEDs, which are typically referred to as “multistring LEDs”.
For driving an array of LEDs, electronic circuits typically employ a converter to transform an AC input voltage (e.g., “AC mains”) and provide a DC voltage source, with a linear regulator then being used to regulate LED current. For example, in Mueler et al., U.S. Pat. No. 6,016,038, entitled “Multicolored LED Lighting Method and Apparatus”, the LEDs may be controlled by a processor to alter the brightness and/or color of the generated light, such as by using pulse-width modulated (“PWM”) signals.
Multistring LED Drivers with PWM regulation are known, e.g., Subramanian Muthu, Frank J. P. Schuurmans, and Michael D. Pashly “Red, Blue, and Green LED for White Light Illumination”, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. 2, March/April 2002, pp. 333-338. Such multistring LED drivers typically require redundant drivers for every LED string. For example, in the system illustrated in FIG. 1, three separate and independent flyback converters 10 operating at a constant switching frequency of 100 kHz drive a corresponding string of an RGB LED light source 15, with a PWM 20 driving scheme operating at a frequency of 120 Hz. Each flyback converter 10 contains a current loop to maintain a constant peak current for the PWM pulses. The color control system is implemented in a DSP controller 25 (TMS320F240), which supplies the PWM turn-on and turn off signals for the power supply.
A similar approach using redundant drivers for multistring LEDs is suggested in Chang et al., U.S. Pat. No. 6,369,525, entitled “White Light Emitting Diode Lamp Driver Based on Multiple Output Converter with Output Current Mode Control,” which utilizes a white LED array driver circuit with a multiple output flyback (or forward) converters with output current mode control, as illustrated in FIG. 2. The circuit 40 comprises a power supply source, and a transformer having a primary winding 41 and multiple secondary windings 42, with each light emitting diode string coupled in the circuit to one of the secondary windings. A main controller 43 is coupled to a first of the light emitting diode strings 46 and is configured to control a flow of current to the primary transformer winding 41. The circuit also comprises an additional plurality of secondary controllers 44, 45, each of which is correspondingly coupled to another light emitting diode string 47, 48 to control a flow of current to its corresponding light emitting diode string.
An analog implementation of this teaching can be found in the AS3691 product from Austriamicrosystems (LEDs Magazine 2005). The AS3691 includes four independent high precision current sources each capable of sinking 400 mA. The operating current per LED channel can be set via an external resistor, while the LED brightness is controlled by four independent pulse width modulated inputs. The AS3691 integrates four independent current sinks per chip, enabling it to drive either four white LEDs each sinking 400 mA or a single white LED with up to 1.6 A.
Another method utilizes multiple, separate linear regulators, with each regulator separately coupled to an LED string of an LED array. For example, an AC-to-DC converter, for transforming AC input voltage into a DC voltage source, is coupled to multiple, dedicated linear regulators, with one regulator coupled to each LED string to regulate the current in that corresponding LED string. This approach represents a multistage power system with low efficiency of power conversion, in addition to already low efficiency of series pass current regulators, particularly when the DC voltage is constant and does not depend on current. This method of multiple and separate linear regulators is illustrated in the following U.S. Pat. Nos. 7,064,498 Light Emitting Diode Based Products; 7,038,399 Methods And Apparatus For Providing Power To Lighting; 6,965,205 Light Emitting Diode Based Products; 6,806,659 Multicolored LED Lighting Method And Apparatus; 6,801,003 Systems And Methods For Synchronizing Lighting Effects; 6,788,011 Multicolored LED Lighting Method And Apparatus; 6,720,745 Data Delivery Track; 6,636,003 Apparatus And Method For Adjusting The Color Temperature Of White Semiconductor Light Emitters; 6,624,597 Systems And Methods For Providing Illumination In Machine Vision Systems; 6,548,967 Universal Lighting Network Methods And Systems; 6,528,954 Smart Light Bulb; 6,459,919 Precision Illumination; 6,340,868 Illumination Components; 6,292,901 Power/Data Protocol; 6,211,626 Illumination Components; 6,166,496 Lighting Entertainment System; and 6,150,774 Multicolored LED Lighting Method And Apparatus.
Similarly, in U.S. Pat. No. 6,016,038, entitled “Multicolored LED Lighting Method and Apparatus,” each LED string of the LED array is controlled by a separate current regulator with a processor, to alter the brightness and/or color of the generated light using pulse-width modulated signals. In this case, an additional, current sink stage is added in series with each LED string current regulator, resulting in a further decrease in efficiency, particularly when the current sink is used to bypass the LED current to ground when the LED should be off. This multistage power system, with dedicated current converters and controllers in each LED channel, in addition to low efficiency, has a large size, many expensive components, and is expensive to manufacture.
Lastly, in Archenhold et al. U.S. Pat. No. 6,963,175, entitled “Illumination Control System,” a light emitting diode illumination control system is disclosed for driving a current circuit for energizing one or more LED light sources. The system comprises a control system including a microprocessor, arranged to control a pulse amplitude modulated (PAM) voltage controlled current circuit, and may employ a monitor for monitoring at least one ambient condition and a microprocessor operable to control the current circuit in response to the monitored conditions. This proposal has several significant problems: (1) the system is very complex, inefficient (for power conversion), has many expensive components, and is expensive to manufacture; (2) the current to emission (color) transfer function in emitting diodes is substantially nonlinear, leading to poor color control or requiring additional, undisclosed technical means for compensation for this nonlinearity (not suggested in the patent); and (3) the disclosed current source operates poorly, suffering from thermal dependency and requiring correction by a microprocessor.
The multi-output or separate power converters and controllers for each LED string of an LED array increases the cost and size of the LED driver, and reduces the functionality and efficiency of the driver. Accordingly, a need remains for a multistring LED driver which utilizes a single power converter and controller for an entire LED array and does not utilize these multiple, separate power converters and controllers for each LED string. Such a multistring LED driver should provide for independent current control for each LED string of the array, for corresponding effective color and brightness control. In addition, such an LED array driver should provide for local LED regulation, providing local compensation of LED emission due to age and drift of functional parameters, temperature changes of the LED junction, LED production characteristics variation, and variations of devices produced by different manufacturers. Such an LED array driver also should be backwards-compatible with legacy LED control systems.